1. Field of the Invention
The present invention relates to a molding method for forming an optical element, such as a glass lens for use in an optical instrument, by precision glass molding.
2. Related Background Art
In recent years there have been made attempts to form an optical element, such as of optical glass, by molding only, without a grinding process. For this purpose most efficient is a method of pouring molten glass material into a mold and forming said glass material by pressurized molding, but such method is not suitable for precision glass molding because of the difficulty in controlling the glass contraction at cooling. For this reason there 18 generally employed a method of forming the glass material into a predetermined form in advance and molding said glass material under pressure and under heating between mold members, as disclosed, for example, in the Japanese Laid-open Patent Sho 58-84134.
In order to obtain a highly precise molded glass product in such method, the form of the molding face of the mold member has to be accurately transferred to the glass material, and, for this purpose, it is particularly important that the molding face of the mold member remain in close contact with the glass material in the course of cooling after the deformation of the glass material. For achieving such close contact, the Japanese Laid-open Patent Sho 60-145919 discloses a method of employing a space defining member of a larger thermal expansion than that of the glass material between the upper and lower mold members.
In the following there will be explained the above-mentioned conventional molding method, with reference to the attached drawings.
FIG. 11 is a schematic cross-sectional view of molding a lens with glass material by the above-mentioned conventional method.
There are shown a molded lens 44; a pair of mold members 41, 42 (upper mold 41 and lower mold 42); a space defining member 43; and a support member 45. After the glass material is supported by the support member 45 and heated close to the softening point of the glass material by a suitable method, said glass material is placed between the upper mold 41 and the lower mold 42, and is pressure molded by an unrepresented pressurizing mechanism. In the cooling step after the molding, contraction occurs in all the members including the molds and the lens. If the space defining member 43 is composed of an ordinary material in the structure shown in FIG. 11, the pressure of the upper and lower molds 41, 42 is not effectively transmitted to the lens 44 since the contraction of glass is larger than that of other members. However this drawback can be avoided by composing the space defining member 43 with a material of a larger thermal expansion than that of the lens 44 and precisely controlling the cooling of said member 43. By maintaining the pressurized state at least to the distortion point of the glass material after the molding step, precisely measuring the temperature of the space defining member and terminating said pressurized state at a predetermined temperature of the space defining member, the glass material is given sufficient pressure from the molding faces of the upper and lower molds 41, 42 to achieve precise transfer of the form of said molding faces, as the space defining member contracts more than the glass material.
However, since such pressurized molding of glass showing thermal expansion is conducted at a temperature at least equal to the yield point or transition point of the glass and since the mold members and the glass show different contacting behaviors as explained above, it is necessary to precisely control the glass contraction at the cooling after molding and to slowly cool the mold members and the molded glass in the prolonged pressurized state.
In addition to the foregoing conventional technology, the Japanese Laid-open Patent Sho 63-182223 discloses a method of maintaining the molding faces of mold members in close contact with the molded glass product in the course of cooling after the molding.
The mechanism employed therein includes, as shown in FIG. 10, an upper mold 71, a lower mold 72, a side mold 73, pressurizing means 76, 77 for pressing said upper and lower molds, and a spacer 75 positioned at the upper end of said side mold 73. The pressurized molding in the above-explained molding apparatus is conducted by placing a glass material 74 between the upper and lower molds 71, 72 and pressurizing the upper and lower molds 71, 72 by the pressurizing means 76, 77 with the spacer 75 on the side mold 73. Then, in preparation for the contraction of glass in the course of cooling, the spacer 75 is eliminated immediately before the cooling and the upper and lower molds 71, 72 are pressurized by the pressurizing means 76, 77 whereby the upper and lower molds 76, 77 can be maintained in close contact with the glass material 74 in the course of said cooling.
However, in the above-explained apparatus in which the contraction of glass is compensated by the thickness of the spacer 75, it is difficult to achieve exact compensation by such spacer 75, since the actual contraction of glass takes place in the order of microns.
Also such methods require a long molding time for achieving precise control of the temperature of the space control member in order to realize precise transfer of the form of the optical face, since the glass undergoes special thermal expansion as explained below.
FIG. 12 illustrates the behavior of thermal expansion of glass, wherein curve a indicates thermal expansion of glass, while curve b indicates that of metal for comparison.
As shown in FIG. 12, glass expands almost linearly as the function of temperature up to the glass transition point, but shows a several times larger expansion beyond said glass transition point. Then, beyond the deformation point, the thermal expansion becomes even larger, but the apparent expansion is no longer observed because the glass starts deformation.
Since the pressurized molding of glass showing above-explained thermal expansion takes place at a temperature above the deformation point or the transition point, and since the molds and the glass show different behaviors of contraction as explained above, it is necessary to precisely control the contraction of glass and to realize gradual cooling by maintaining the molds and the glass in the pressurized state for a long period in the course of cooling after molding.